The present invention relates broadly to antenna diagnostics and more particularly to a new holographic method for use in antenna diagnostics.
Holographic methods for antenna diagnostics are extensively used to verify the excitation or the detection of failure of individual elements in antenna arrays and to determine the surface accuracy of antenna reflectors used in radiotelescopes.
Typically, these applications of holography are solely performed at the frequency of operation. The field measurements are performed at the central frequency of the operating bandwidth and eventually also at the edges of the frequency band. Holography in the frequency domain is equivalent to restricting attention to the electrical properties of antennas at a single frequency and consequently the extent of information provided by the frequency domain holography is limited. The complete behaviour of the antenna is fully described only when the entire frequency response is taken into account. This considerably increases the testing time.
Further, the use of large satellite constellations is a common trend in current telecommunication and navigation projects, e.g. Skybridge and GalileoSat. The large number of antennas required immediately calls for faster testing methods, but not at the expense of accuracy: a reasonable trade-off should always be commercially accepted.
Another issue that increases testing time is the frequency bandwidth over which the antennas have to be operating. This is the case for antennas for ultra-wide band radars, which typically have bandwidths in excess of 20%. In addition, the potential radiation interference of antennas with neighbouring instruments on board a spacecraft requires also measurements out of the operational bandwidth, which implies larger test campaigns, e.g. antennas on board Envisat. Generally, the testing of such antennas requires reconfiguration of the measurement range several times to cover the desired bandwidth. Consequently, the measurement time and cost increase. The testing time for traditional measurements may be too large and, consequently too expensive, from an industrial point of view. Therefore, there is a need for faster and less expensive antenna measurement capabilities.
Time domain measurements have the advantage of determining the wideband frequency response of an antenna under test. Therefore, they are very fast, not only because of the shorter measurement time required, but also because the radiation pattern over a large number of frequencies can be directly retrieved from the same measurement. To achieve the same performance with conventional frequency domain instrumentation would require about ten times the measurement time needed in the time domain, if high averaging is applied and the same number of frequencies is measured.
The unique features of time domain measurements are exploited in accordance with this invention in the time domain antenna holography. The information obtained from this new holographic approach offers therefore much wider insight into the antenna electrical characteristics than a measurement at a single frequency. In fact, this involves the reconstruction of the time-varying far field distribution across an aperture from direct time domain measurements.
It is an object of this invention to provide a novel, fast method and system which permits use of time domain holography for antenna diagnostics.
Another object of the invention is to provide a novel method which permits the reconstruction of time-varying far field distribution across the antenna aperture from direct time domain measurements.
In accordance with this invention, the antenna under test is placed and aligned in an antenna test range and the antenna or the range antenna is excited with an ultra-short duration voltage pulse and the antenna voltage radiation pattern in the time domain is measured using a digitising oscilloscope connected to the antenna under test or the range antenna. The measured radiation pattern, if necessary, is converted to far field. The resulting time-varying far field distribution across the antenna aperture is then reconstructed using time domain holography. Electrical properties of the antenna are directly retrieved from direct analysis of the holographic plot.
A wide range of electrical properties of an antenna, such as element failure diagnostics in array antennas and details of the feed network design, can be directly retrieved from direct analysis of the time-varying far field distribution.
The approach of the invention thus provides unique possibilities for analysis propagation effects, reverse engineering of array feed networks and substrate electrical properties.